This invention relates generally to computed tomography (CT) imaging and more particularly, to determining x-ray beam position in a multi-slice CT system.
In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts that attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Known CT scanners include a pre-patient collimating device having an aperture that defines the x-ray beam profile in the z-axis (patient axis). When performing a scan, the x-ray beam typically moves up to 2 mm in the z-direction on tile detector array due to thermal, gravitational, and centrifugal force effects. This movement of the fan beam affects the signal strength at the detector, which results in artifacts in a reconstructed image.
Known CT scanners perform data corrections to compensate for detector signal variation as a function of z-axis x-ray beam position on the detector. More particularly, multi-slice CT systems typically utilize a precise closed loop z-axis tracking system to minimize beam motion and to perform z-axis corrections to compensate for z-axis beam motion and better utilize x-ray dosage. Known z-axis beam position sensing devices, or z-axis offset detectors, include a metal wedge or series of alternate wedges that are placed over one or more detector channels to induce a significant and repeatable signal variation as a function of the z-axis position. A detailed description of detecting fan beam positions by using known wedges is described, for example, in U.S. Pat. No. 4,559,639, entitled xe2x80x9cX-Ray Detector with Compensation for Height-Dependant Sensitivity,xe2x80x9d assigned to the present assignee and incorporated herein by reference.
Although the known z-axis beam position sensing devices provide acceptable results, e.g., artifact reduction, it would be desirable to increase beam position measurement sensitivity and accuracy to further improve artifact reduction. It also would be desirable to improve artifact reduction without significantly increasing the system cost and processing time.
These and other objects may be attained in a system for determining x-ray beam position by utilizing signals from detector data or z position cells to generate difference, or ratio, signals representative of beam position. Such difference or ratio signals can then be used to control the pre-patient collimator so that if the beam is out of alignment, the beam is brought back into alignment by the collimator. The present invention is particularly applicable in multi-slice computed tomography systems, including two and four slice systems.
In a two slice system, for example, a collimated x-ray beam is projected toward two adjacent first and second detector cells. A plane, generally referred to as the xe2x80x9cfan beam planexe2x80x9d, contains the centerline of focal spot and the centerline of the beam. When the beam is positioned in its most desirable orientation, the fan beam plane is aligned with the centerline Do of the exposure area on the adjacent detector cells.
The signal intensity A of the signal output by the first detector cell and the signal intensity B of the signal output by the second detector cell are related to the position of the focal spot. Specifically, the z position of the centerline of the fan beam can be determined by relating the signal intensities A and B according to the ratio [(Axe2x88x92B)/(A+B)]. Such ratio is representative of the beam location and can be used to control adjustment of the collimator to maintain the beam in the desired position.
The above described system has a high sensitivity to focal spot movement and generates a signal accurately representative of focal spot position. Such high sensitivity and accuracy facilitates improving artifact reduction. Further, such improved artifact reduction can be achieved without significantly increasing the system cost and processing time.